Thackery comes from a rather well off hog raising family that is located in a village that does a lot of business with dwarves(they love their bacon, am I right?).
The dwarf hold was busy dealing with an enemy force and couldn't spare any defense for the small village so they but together a militia to deal with the small force of raiders.
Thackery, being the second oldest, took up grammy's sword and joined up.
The old blade needed some TLC so the resourceful boy used what he had and got the blade polished up nice with some lard from his family's Red Hog farm.
The repelling of the raiders took a month or two but was a resounding success. So much so that the militia located where the raiders have been holding up and finally getting with a small regiment of dwarves, took the fight to the raiders and erased them from the map.
All told this whole venture took no longer than year.
Thackery, happily, returns home and resumes work on the farm. He had some many stories to tell about the new raiders dagger that he pulled from a chieftain's body.
But he wasn't really happy. Was lost in thought a lot, staring into the middle distance for minutes on end. He loved his family, and his work, but he wanted to do more. He felt like he could do more out there again.
His family noticed this and secretly ordered a fine ironwood shield, bound in dark iron, to be made by the dwarven hold nearby.
They even ordered a red boar rampant to be painted onto the face, edged in red gold.
One night, they had a big chicken dinner and presented Thackery with the shield and grammy's sword and told him to go be hero he so wanted to be. He had their blessing so he hit the road.
While on the road he decided to take the surname Militia because he hated being only thought of as a Farmerson.
I think this is how he finds his way to Sandpoint, just wandering on the roads and hearing about the festival.